The corrosion of industrial piping and other components such as valves and pumps is a major problem in some industries. The oil industry, in particular, faces severely corrosive environments, with corrosive gasses and liquids such as H2S (hydrogen sulfide) at elevated temperatures and pressures. Additionally, these conditions form severe wear and erosion environments. One solution to these issues is to coat a lower grade base material with a high quality coating material having the desired high corrosion and wear-resistant properties. Typically, these types of properties will be found in metal, ceramic and particularly diamond-like carbon coatings.
Stainless steel is one example of a metal alloy that is sometimes coated to improve corrosion resistance. Other expensive specialty alloys, such as Hastelloy and Inconel (both of which are federally registered trademarks of Huntington Alloys Corporation), are commonly used for exhaust piping in not only the semiconductor industry, but in chemical processing industries in general. These alloys exhibit high temperature strength and corrosion resistance. Again, a less expensive base material can be used if a suitable surface coating is applied to the interior surface that is to be exposed to the corrosive environment.
In the application of a corrosion-resistant coating to a pipe or other workpiece, adhesion of the coating material to the workpiece must be considered. For a particular coating material, some base materials more readily adhere to the coating material than others. For example, a coating material of diamond-like carbon (DLC) adheres more readily to smooth stainless steel than to either nickel or a rough surface such as carbon steel. Chemical vapor deposition (CVD) is used in numerous applications in which adhesion and corrosion resistance are critical performance parameters. Historically, adhesion of a coating bonded to a substrate or other workpiece is promoted by careful selection of the activation energy for bonding, selection of temperature, and the application of surface area preparations. Plasma enhanced CVD (PECVD) enables depositing films at reduced temperatures, but the energy delivered by plasma typically is not sufficient to provide the desired level of adhesion.
U.S. Pat. No. 5,965,217 to Sugiyama et al. describes forming two intermediate films prior to forming the DLC film. The first intermediate film is selected for its ability to adhere to the workpiece itself, while the second intermediate film is selected for its ability to adhere readily to the subsequently formed DLC film. The first intermediate film is provided by sputtering or resistance heating evaporation using an auxiliary electrode formed of the selected material. The patent states that the first intermediate film may be titanium, chromium, aluminum, or a metal silicide film such as a titanium-silicon alloy, a carbon-silicon alloy, a titanium-germanium alloy or a chromium-germanium alloy. In comparison to the process for forming the first intermediate film, the second intermediate film is formed by supplying a gas which contains silicon or germanium into a vacuum vessel to produce a plasma. The purpose of the two intermediate films is to promote adhesion, while the purpose of the DLC layer is to promote corrosion resistance. Nevertheless, the concern is that if the workpiece is intended for use in contact with corrosive fluids, a pinhole through the DLC will allow the corrosive flow to reach the intermediate layers, potentially resulting in an undercut of material.
The coating process for steel and other conductive workpieces continues to be an issue, particularly if the workpiece is intended for regular contact with corrosive fluids. Techniques used in other industries are of interest, but do not provide a complete solution. U.S. Pat. No. 6,086,796 to Brown et al. describes techniques for coating a recording medium, such as a phase-change optical medium for use with laser optics. The lifetime of the phase-change recording medium can be extended by forming DLC on a germanium-containing adhesion-promoting interlayer. The patent states that the DLC may be formed using ion beam deposition, but other techniques may be employed. The focus of the interlayer is to promote adhesion, rather than to retard corrosion, since the recording medium is not exposed to aggressive environment or rough substrates. Additionally, the deposition rate for ion beam deposition may not be suitable for coating workpieces such as pipes. “Example 1” in Brown et al. describes deposition of a DLC layer having a thickness of 150 Angstroms in 5.5 minutes.
While prior art approaches operate well in many applications for coating a workpiece, further advances are sought.